Petroleum and natural gas produced from oil wells and gas wells contain corrosive gas such as carbon dioxide gas and hydrogen sulfide gas. In such a wet carbon dioxide gas environment, martensitic stainless steel pipes having high corrosion resistance are used as oil country tubular goods. More specifically, 13Cr stainless steel pipes, typically API13Cr steel pipes are widely used. The 13Cr stainless steel pipe is resistant to carbon dioxide gas corrosion as it contains about 13% Cr and martensitic in structure as it contains about 0.2% C.
In recent years, deeper oil and gas wells have been explored and developed. An oil country tubular good (hereinafter, simply referred to as OCTG) for use in a deep well in a wet carbon dioxide environment must have a high strength equal to 655 MPa or more and high toughness. In a wet carbon dioxide gas environment at high temperatures in the range from 80° C. to 150° C., there is a concern that an active path corrosion type stress corrosion cracking (hereinafter simply as “SCC”) may be generated, and therefore high SCC resistance is requested.
The following disadvantages are encountered when a 13Cr stainless steel pipe is used in a deep well in a high temperature wet carbon dioxide gas environment.
(1) For its high C content, necessary toughness cannot be obtained if the strength is raised to 655 MPa or more.
(2) The 13Cr stainless steel pipe is subjected to quenching and tempering in the manufacturing process, and Cr carbides 50 are formed in the structure after the tempering as shown in FIG. 1. A Cr-depleted region 60 as a low Cr content region forms in the periphery of the Cr carbide 50 or at a grain boundary. The Cr-depleted region 60 increases the SCC susceptibility. Therefore, the 13Cr stainless steel pipe having the Cr-depleted region 60 does not have SCC resistance necessary for use in a deep well in a high temperature wet carbon dioxide environment.
This is why the super 13Cr martensitic stainless steel pipe usable in a deep well in a high temperature wet carbon dioxide environment has been developed. The super 13Cr martensitic stainless steel pipe has higher SCC resistance than that of the 13Cr stainless steel pipe because of a passive film on the surface formed by adding an alloy element such as Mo and Cu and its C content set to 0.1% or less. This is because almost no Cr carbide is precipitated in the structure after the tempering for the low C content as shown in FIG. 2, provided that the tempering condition is properly set.
Since a large quantity of Ni as an austenite-forming element is contained in place of C that is also an austenite-forming element, the martensitic structure can be kept, even if the C content is low. Therefore, the super 13Cr martensitic stainless steel pipe has high strength and toughness necessary for use in a high temperature wet carbon dioxide gas environment.
The conventional 13Cr martensitic stainless steel pipe is subjected to quenching and tempering in order to obtain desired strength, but a 13Cr martensitic stainless steel pipe produced without the tempering following rolling (hereinafter referred to as “tempering-omitted martensitic stainless steel pipe”) has been developed for reducing the manufacturing cost. The tempering-omitted martensitic stainless steel pipe is disclosed by JP 2003-183781 A, JP 2003-193203 A, and JP 2003-129190 A. According to these publications, desired strength and toughness can be obtained, even if the tempering is omitted.
However, the inventors have found through examinations that the tempering-omitted martensitic stainless steel pipe has SCC resistance lower than that of the conventional super 13Cr martensitic stainless steel pipe. As shown in FIG. 3, a Cr-depleted region is not produced on the inner side than a region about as deep as 100 μm from the surface of the tempering-omitted martensitic stainless steel pipe, but a Cr-depleted region 60 is generated in a region from the surface to a depth of about 100 μm.
The Cr-depleted region 60 under the surface forms after hot working. More specifically, the Cr-depleted region 60 forms when mill scales form after rolling and Cr under the surface is absorbed in the mill scales, or a Cr carbide 50 forms under the surface because of graphite used as a lubricant for the rolling, so that the Cr-depleted region 60 forms around the Cr carbide 50. The conventional super 13Cr martensitic stainless steel pipe is subjected to tempering after rolling, and therefore such a Cr-depleted region 60 under the surface is eliminated during the tempering process, but the tempering-omitted martensitic stainless steel pipe is produced without being subjected to the tempering, and therefore many Cr-depleted regions 60 should be left unremoved under the surface.
The tempering-omitted martensitic stainless steel pipe disclosed by JP 2003-193204 A has high SCC resistance. However, in the tests for evaluating the SCC resistance in the disclosure, a smooth test piece, i.e., a test piece having a polished surface was used. More specifically, the SCC resistance was not evaluated using a test piece including a Cr-depleted region under the surface. The inventors conducted SCC tests using test pieces including a Cr-depleted region under the surface according to the disclosed condition and found that the SCC resistance of the test pieces including a Cr-depleted region under the surface was lower than that of the smooth test piece.
Therefore, if the tempering-omitted martensitic stainless steel pipe including many Cr-depleted regions under the surface is used in a deep well in a high temperature wet carbon dioxide gas environment, SCC could be generated.
As a method of removing such Cr-depleted regions under the surface, shot-blasting and/or pickling may be carried out. These kinds of processing however increase the manufacturing cost. Even after these kinds of processing, there is still a possibility that Cr-depleted regions under the surface may remain unremoved depending on the processing condition.